This invention relates to an improved form of filter, and in particular to a filter having a novel means of clearing contaminants from the upstream or filtration side of the filter to thereby increase filtering efficiency.
The present invention will have many varied and diverse applications. The scope of the invention will include the filtering of solids from liquids, the filtering of solids entrained in air, the filtering of immiscible liquids from process liquids, and the filtering of droplets from air.
The filtration of contaminants from fluids occurs in many applications including: industrial, agricultural, medical, food, fibre, effluent treatment, by-product recovery and fluid recycling. For example, in a washing or cleaning process, solids are present in the used fluid and provided that those solids can be removed, the fluid can then be repeatedly reused in the process until requiring eventual disposal.
There are many known conventional filtering techniques, many of which are referred to as xe2x80x9cdead-endxe2x80x9d filtration. In these conventional xe2x80x9cdead-endxe2x80x9d filtration processes, as fluid passes through the filter medium, suspended contaminants larger than the pore or aperture size of the filter medium are collected on the upstream side of the filter medium as process fluid passes through the filter medium. As the amount of contaminant accumulates on the upstream side of the filter medium, the resistance to flow through the filter increases, the flow of fluid passing through the filter medium decreases and the differential pressure across the filter medium increases. This continues until a situation is reached where the accumulation of contaminants on the upstream side of the filter medium has to be removed to enable the filtration process to continue. This xe2x80x9cdead-endxe2x80x9d point in conventional filtration systems is the reason why some form of cleaning is required. Typical cleaning processes include: periodic back flushing where the direction of fluid flow through the filter medium is reversed; and mechanical scraping where a scraper traverses the surface of the filter medium to remove contaminants thereby exposing the filter medium to damage. These cleaning processes interrupt the filtering process and therefore introduce further inefficiencies. Furthermore, in some applications it is impractical to clean the filter medium frequently enough to prevent build up of contaminant on the filter medium surface using those cleaning processes and therefore poor filtration efficiencies result.
While there exist numerous types of self-cleaning filters their limitations include: the types of fluids that they are able to filter, their durability, the flow rates achievable and the operational time of the filtration process before shutdown maintenance is required.
It is an object of this invention to overcome the abovementioned problems and to produce a filtration system which more readily and continually clears accumulating contaminants from a filter medium during a filtering process.
In its broadest form, the invention is a filter system comprising:
a filter medium, for filtering contaminants from a process fluid, having a filtration side on which said contaminants collect and a filtrate side from which filtrate flows;
a counter-flow generator located on said filtrate side that directs a localised stream of counter-flow fluid from said filtrate side to said filtration side of said filter medium to thereby dislodge said contaminants from said filtration side of said filter medium, wherein said localised stream traverses a substantial proportion of said filter medium.
Causing the contaminants that accumulate on the filtration side to dislodge has several advantages. There is a breaking up of any caking that occurs on the filtration side, and the contaminants are dispersed into the process fluid thereby greatly reducing their concentration in the immediate vicinity of the filter medium when the flow again returns through the area of the filter medium from which contaminants have been dislodged. This has the effect of keeping the pressure differential across the filter medium low. Advantages arising from this form of continuous maintenance of filter medium include: elimination of mechanical damage to the filter medium, reduced filter area requirement, limited pressure build-up across the filter media and, importantly, reduced forces on the filter medium itself. Reduced forces on the filter medium allow additional versatility in the selection of filter media and reduce the need for structural supports for these filter media. A unitary filter medium only supported at its edges may be used, wherein the filter is unitary in the sense that it is uniform in porosity and is homogenous.
Any suitable filter medium may be used. The filter medium may be rigid or flexible and may be made from a range of woven or unwoven porous type materials such as fibrous cloth or sheet, steel or polymeric open mesh type material. Filter media with relatively large pore sizes such as screens or sieves may also be used.
Preferably the entire filter medium less only the area through which the localised stream of counter-flow flows, is continuously available for filtering contaminants. This produces a more efficient and more compact filtration system.
Preferably the filtration system further comprises a clearing-flow generator located on the filtration side that directs a localised stream of clearing-flow with a velocity component having a direction across an area on the filter medium adjacent the area at which the localised stream of counter-flow flows through the filter medium. Furthermore, preferably the localised clearing-flow stream moves in unison with the localised counter-flow stream during at least a portion of said counter-flow stream""s traverse.
The operation of the counter-flow generator (with or without the clearing-flow generator) may be either periodic or continuous.
In a first preferred arrangement the filter system has a filter medium that is substantially planar and is inclined to the horizontal. In this arrangement the process fluid flows downwards from the upper end of the top side of the inclined filter medium and the counter-flow generator comprises at least one fluid outlet orientated to direct counter-flow fluid towards the filter medium across the width of the filter medium and movable in a direction substantially parallel to the filtrate side of the filter medium to continuously traverse a substantial proportion of the filter medium. Underneath the filter medium there may be provided a filtrate collection tray. Preferably a clearing-flow generator as described above is also provided.
This first preferred arrangement is particularly suitable for continuous filtration of process liquids with a high proportion of contaminant.
In a second preferred arrangement the filter medium is substantially cylindrical in shape and the counter-flow generator further comprises a manifold with a plurality of fluid outlets, the manifold being rotatable about an axis within the substantially cylindrical filter medium to thereby provide a means for traversing the filter medium. Preferably a clearing-flow generator as described above is also provided.
In another aspect of this invention applicable to this second preferred arrangement the filter medium is divided into a grid of elements the centres of which bow towards the direction of flow of the filtrate upon sufficient pressure differential across the filter medium and, when exposed to the localised counter-flow stream, invert to bow in the direction of the counter-flow fluid to thereby assist in the dislodgment of the contaminants from the filter medium.
The above described second preferred arrangement works particularly well when the filter system is located within a volume of process fluid which requires filtration. This can be achieved by immersing the filter medium in a vessel containing the process fluid. The vessel may be a settling tank or pond for instance.
In a third preferred arrangement the filter system according to the second preferred arrangement is enclosed by a substantially cylindrical vessel having a process fluid inlet orientated so as to provide the process fluid with a velocity having a component tangential to the substantially cylindrical filter medium.
Preferably the xe2x80x9cprocess fluidxe2x80x9d inlet is located at the upper end of the enclosing cylindrical vessel and preferably there is a contaminant discharge point at the lower end of the enclosing cylindrical vessel. With this arrangement contaminants are subject to centrifugal forces which assist to move them radially away from the filter medium.
The counter-flow generator, as previously described for each of the three preferred arrangements, preferably comprises a focused spray jet or a series of focused spray jets which direct a high velocity counter-flow stream towards the filter medium.
Filtration systems according to the invention are able to operate without or with process control systems.
The term xe2x80x9cprocess fluidxe2x80x9d is used throughout this specification and its claims to denote fluid to be filtered. It can be fluid from any source and is not restricted to fluid from any specific source or group of sources.
The word xe2x80x9ccontaminantxe2x80x9d is used throughout this specification and its claims to denote the component of the process fluid that is to be filtered from the process fluid. In some applications the contaminant may be a valuable by-product that can be used after filtration.
In order to fully understand the invention, preferred embodiments will now be described, but it will be realised that the invention is not to be confined or restricted to the precise nature of these embodiments.